1. Field of the Invention
The present invention relates to methods of producing anodes for solid oxide fuel cells.
2. Description of the Related Art
Solid oxide fuel cells using oxygen-ion conductive solid oxides as electrolytes are operated at temperatures of 700° C. or higher. Due to this high-temperature operation, solid oxide fuel cells can exhibit the highest efficiency among other types of fuel cells. All elements of solid oxide fuel cells are made of solids. The structural characteristics make solid oxide fuel cells simpler in structure than any other fuel cell, do not cause problems of degradation or loss and corrosion of electrolytes, and enable direct supply of fuel through internal structure modification without using any noble metal catalysts. Another advantage of solid oxide fuel cells is that combined heat and power generation is possible using very hot exhaust gases. Due to these advantages, research aimed at commercialization of solid oxide fuel cells in the early 21st century is being actively undertaken in advanced countries, including U.S., Japan and Germany.
A general solid oxide fuel cell includes an oxygen ion-conducting dense electrolyte layer, a porous cathode and a porous anode. The two electrodes are positioned at both sides of the electrolyte layer. 8 mol % yttria (Y2O3)-stabilized zirconia (ZrO2) (YSZ), LaSrMnO3 (LSM) and Ni/YSZ are mainly used as materials for the electrolyte, the cathode and the anode, respectively.
The operational principle of the solid oxide fuel cell is as follows. Oxygen receives electrons and is reduced to oxygen ions at the porous cathode. The oxygen ions reach an electrolyte/cathode interface and migrate to the anode through the dense electrolyte layer. The oxygen ions react with hydrogen supplied from the porous anode to create water. When the anode where electrons are produced is connected to the cathode where electrons are consumed, a current of electricity flows through the two electrodes.
Reactions of the anode take place at the three-phase boundary (TPB) where fuel (for example, H2), the Ni catalyst and the YSZ meet together. The performance of the fuel cell can be maximized by optimizing the relationship between an increase in gas diffusion through the anode and a drop in performance resulting from the replacement of the TPB with pores. The porosity of the Ni/YSZ anode produced without using any pore former reaches 23 to 27%, which varies depending on the amount of NiO mixed from 56 to 70% by weight. However, this porosity level is not sufficient for gas diffusion, leading to a drop in the performance of the fuel cell. Thus, a further increase in porosity is needed.
For maximum performance of the fuel cell, the anode is required to have a uniform fine structure and a uniform porous structure, which are advantageous for gas diffusion. A uniform fine structure of the anode is obtained using a homogeneous mixture of fine powders. However, in the case where the fine powders is smaller in size than 1 μm, micropores are formed upon sintering and the number of open pores is not sufficient, making it difficult for gases to diffuse through the anode. Thus, the addition of a suitable pore former is needed to form a sufficient number of open pores.
Carbon powders are usually used as pore formers. Other pore formers include organic materials, such as fine polymethyl methacrylate (PMMA) beads and starches. There is a study reporting that when a carbon powder was used in amounts ranging from 20 to 50% by volume, 30% by volume of the carbon powder provided a porosity of 35%, which is a preferable level. Like the study on the use of the carbon powder, some studies reported that the addition of a large amount of rice or corn starch produced a porosity of 30 to 40%.
According to a prior art method, an anode support is produced using a mixture of a carbon-based pore former (such as carbon black or a polymer), which is mentioned above, a matrix material (such as NiO/YSZ), a binder and one or more additives.
However, the use of the carbon-based pore former as a material for the production of the anode support is environmentally harmful. The carbon-based pore former is very susceptible to external factors, such as molding pressure, during subsequent molding. This susceptibility makes it difficult to control pores. Particularly, the carbon-based pore former is difficult to uniformly mix with the matrix material and tends to leave aggregates because it is not readily dispersed in water and has a very different density from the matrix material. Further, the carbon-based pore former emits heat when being oxidized, resulting in a local temperature rise. As a result, the anode support is likely to be highly defective and has a high shrinkage.